The long-term research objective is to design, synthesize and investigate model compound systems, which can help elucidate fundamental aspects of structure, metal-ligation, spectroscopy and reactivity relevant to the chemistry utilized by heme-copper oxidases (e.g., cytochrome c oxidases (CcOs)) and nitric oxide reductases (NORs). These evolutionarily related enzymes are involved in the bioenergetics of aerobic and anaerobic organisms, and have in common a heme/M (M = Cu or non-heme Fe) active site, which reductively cleaves dioxygen (O2) or nitric oxide (NO), respectively. The research can contribute to a better understanding of enzyme structure and mechanism, and provide fundamental insights into biological O2 activation, NO and nitrogen oxide chemistry and biochemistry, and issues related to nitrogen oxides in the environment. Major themes are the synthesis and characterization of discrete heme/M compounds, their reactivity studies with O2 and NO, the coordination chemistry and the photochemistry of heme/M complexes, and the reductive O-O bond cleavage of heme/Cu/O2 adducts. Specific aims include (1) further characterization and elucidation of the peroxo-connectivity and electronic structures of existing heme-peroxo- Cu complexes, (2) expanded studies on heme/Cu/O2 chemistry employing various tridentate Cu-ligands which include systems with a cross-linked imidazole-phenol moiety, (3) synthesis and characterization of low-spin heme-peroxo-Cu complexes, (4) investigation and elaboration of the chemistry of NOR model systems which produce nitrous oxide (N2O), (5) study of nitric oxide reactivity with heme/Cu complexes, (6) development of the coordination chemistry of heme-O(H)-M systems, (7) investigation of atom/ligand/electron transfer in heme/M complexes induced by photochemistry, and (8 & 9) study of reductive O-O bond cleavage of heme/Cu/O2 adducts either by controlling H+, e-, and/or H. sources, or by employing a cryoreduction, gamma-irradiation technique.